Moon jellies (Aurelia aurita) are translucent, saucer-shaped jellyfish found globally in coastal waters, from temperate to tropical seas. In recent decades, large aggregations, or “blooms,” of moon jellies have become increasingly prevalent worldwide. This rise in populations has prompted investigations into the underlying causes of this ecological shift.
Moon Jelly Biology
Moon jellies possess specific biological characteristics that contribute to their capacity for rapid population growth. Their life cycle includes free-swimming medusae and sessile polyps. Polyps attach to hard surfaces, reproducing asexually through budding or by strobilation, releasing tiny ephyrae. This dual reproductive strategy, combining sexual and asexual reproduction, allows for efficient population expansion.
The polyps can persist in environments that may be unfavorable for adult medusae, acting as a dormant reservoir for future blooms. Moon jellies are also generalist feeders, consuming a wide range of plankton, including small crustaceans, fish eggs, and larvae. This adaptable diet means they are less susceptible to food scarcity compared to more specialized predators, further supporting their ability to thrive and increase in numbers across diverse marine conditions.
Environmental Factors
Several broad environmental changes are contributing to the increase in moon jelly populations. Rising ocean temperatures, a consequence of global warming, accelerate moon jelly growth and reproduction. Warmer waters lead to more frequent strobilation from polyps, increasing ephyrae released into the water column.
Another significant factor is increased nutrient runoff from land, a process known as eutrophication. This influx of nutrients fuels algal blooms, which in turn provide more food for the zooplankton that moon jellies consume. Eutrophication can also lead to ocean deoxygenation, creating hypoxic (low-oxygen) conditions. Moon jellies exhibit a strong tolerance for low dissolved oxygen levels, thriving in environments where many fish species struggle or are forced to leave, giving them a competitive advantage.
Human Activities
Specific human activities also directly and indirectly contribute to the proliferation of moon jelly populations by altering marine ecosystems. Overfishing is a major factor, as it reduces the populations of fish that compete with jellies for planktonic food sources. Many fish also prey on jellyfish or their larval stages, so their decline can remove natural controls on moon jelly numbers.
Coastal development, including the construction of artificial structures like piers, docks, and breakwaters, provides new attachment sites for moon jelly polyps. These structures offer stable, shaded environments where polyps can thrive and release ephyrae, effectively expanding their habitat. Marine pollution, while often detrimental to marine life, can sometimes benefit jellies by disproportionately affecting their competitors or predators, further shifting the ecological balance in favor of these adaptable creatures.
Ecological Implications
The increasing frequency and size of moon jelly blooms carry various ecological implications. Large aggregations of moon jellies can compete with fish, including commercially important species, for shared food resources like zooplankton. This competition can reduce the food available for fish, potentially impacting their growth and survival. Moon jellies also consume fish larvae, which can negatively affect fish recruitment and overall fish populations.
Beyond ecological impacts, moon jelly blooms pose challenges for human activities. Massive blooms can clog fishing nets, leading to reduced fish catches and economic losses for fisheries. They can also interfere with aquaculture operations by consuming farmed fish larvae or stressing adult fish. Dense aggregations near coastlines can affect tourism by making swimming areas less appealing and potentially clogging water intakes for coastal power plants, causing operational disruptions.